In computer science, the expressions code as data and data as code refer to the interchangeable nature of code and data. Specifically, "code is data" refers to the idea that
source code
In computing, source code, or simply code, is any collection of code, with or without comment (computer programming), comments, written using a human-readable programming language, usually as plain text. The source code of a Computer program, p ...
written in a
programming language
A programming language is a system of notation for writing computer programs. Most programming languages are text-based formal languages, but they may also be graphical. They are a kind of computer language.
The description of a programming l ...
can be manipulated as data, such as a sequence of characters or an
abstract syntax tree
In computer science, an abstract syntax tree (AST), or just syntax tree, is a tree representation of the abstract syntactic structure of text (often source code) written in a formal language. Each node of the tree denotes a construct occurring ...
(AST), and it has an
execution
Capital punishment, also known as the death penalty, is the state-sanctioned practice of deliberately killing a person as a punishment for an actual or supposed crime, usually following an authorized, rule-governed process to conclude that t ...
semantics only in the context of a given
compiler
In computing, a compiler is a computer program that translates computer code written in one programming language (the ''source'' language) into another language (the ''target'' language). The name "compiler" is primarily used for programs that ...
or
interpreter. The expression "data is code" refers to the idea that an arbitrary data structure such as a list of integers can be interpreted or compiled using a specialized language semantics. The notions are often used in the context of
Lisp
A lisp is a speech impairment in which a person misarticulates sibilants (, , , , , , , ). These misarticulations often result in unclear speech.
Types
* A frontal lisp occurs when the tongue is placed anterior to the target. Interdental lispi ...
-like languages that use
S-expressions
In computer programming, an S-expression (or symbolic expression, abbreviated as sexpr or sexp) is an expression in a like-named notation for nested list (tree-structured) data. S-expressions were invented for and popularized by the programming la ...
as their main syntax, as writing programs using nested lists of symbols makes the interpretation of the program as an AST quite transparent (a property known as
homoiconicity
In computer programming, homoiconicity (from the Greek language, Greek words ''homo-'' meaning "the same" and ''icon'' meaning "representation") is a property of some programming languages. A language is homoiconic if a program written in it can b ...
).
These ideas are generally used in the context of what is called
metaprogramming
Metaprogramming is a programming technique in which computer programs have the ability to treat other programs as their data. It means that a program can be designed to read, generate, analyze or transform other programs, and even modify itself ...
, writing programs that treat other programs as their data.
For example, code-as-data allows the
serialization
In computing, serialization (or serialisation) is the process of translating a data structure or object state into a format that can be stored (e.g. files in secondary storage devices, data buffers in primary storage devices) or transmitted (e ...
of
first-class functions
In computer science, a programming language is said to have first-class functions if it treats functions as first-class citizens. This means the language supports passing functions as arguments to other functions, returning them as the values from ...
in a portable manner. Another use case is storing a program in a string, which is then processed by a compiler to produce an executable.
[ More often there is a reflection API that exposes the structure of a program as an object within the language, reducing the possibility of creating a malformed program.
In ]computational theory
In theoretical computer science and mathematics, the theory of computation is the branch that deals with what problems can be solved on a model of computation, using an algorithm, how efficiently they can be solved or to what degree (e.g., ...
, Kleene's second recursion theorem
In computability theory, Kleene's recursion theorems are a pair of fundamental results about the application of computable functions to their own descriptions. The theorems were first proved by Stephen Kleene in 1938 and appear in his 1952 ...
provides a form of code-is-data, by proving that a program can have access to its own source code.
Code-as-data is also a principle of the Von Neumann architecture
The von Neumann architecture — also known as the von Neumann model or Princeton architecture — is a computer architecture based on a 1945 description by John von Neumann, and by others, in the '' First Draft of a Report on the EDVAC''. Th ...
, since stored programs
A stored-program computer is a computer that stores program instructions in electronically or optically accessible memory. This contrasts with systems that stored the program instructions with plugboards or similar mechanisms.
The definition ...
and data are both represented as bits in the same memory device.[ This architecture offers the ability to write ]self-modifying code
In computer science, self-modifying code (SMC) is code that alters its own instructions while it is executing – usually to reduce the instruction path length and improve performance or simply to reduce otherwise repetitively similar code ...
. It also opens the security risk of disguising a malicious program as user data and then using an exploit
Exploit means to take advantage of something (a person, situation, etc.) for one's own end, especially unethically or unjustifiably.
Exploit can mean:
* Exploitation of natural resources
*Exploit (computer security)
* Video game exploit
*Exploita ...
to direct execution to the malicious program.declarative programming
In computer science, declarative programming is a programming paradigm—a style of building the structure and elements of computer programs—that expresses the logic of a computation without describing its control flow.
Many languages that a ...
, the data-as-code principle is often applied. For example, configuration scripts
In computing, configuration files (commonly known simply as config files) are files used to configure the parameters and initial settings for some computer programs. They are used for user applications, server processes and operating system s ...
, domain-specific languages and markup language
Markup language refers to a text-encoding system consisting of a set of symbols inserted in a text document to control its structure, formatting, or the relationship between its parts. Markup is often used to control the display of the document ...
s are cases where program execution is controlled by data elements that are not clearly sequences of commands.
References
Programming language topics
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